The present invention relates to an improvement for length measuring devices of the type which comprise a carrier body which defines a longitudinal groove extending between two ends of the carrier body, a measuring band which defines two ends and is disposed in the groove, and a scanning unit guided for movement along the measuring band to scan the measuring band and thereby measure the relative position of the scanning unit with respect to the measuring band.
A wide variety of length measuring devices are known to the prior art. For example, the devices disclosed in German Pat. No. 27 12 421 and in U.S. Pat. No. 3,816,002 both include a measuring scale which is realized as a flexible band on which a measuring graduation has been applied. In German Pat. No. 27 12 421 it is disclosed that the measuring band is fastened within a groove in a carrier body shaped to carry the band. This carrier body is secured by means of fastening elements to one of the objects to be measured, such as a machine slide piece for example. In the device shown in U.S. Pat. No. 3,816,002 the measuring band is suspended between two clamping points, and it passes through the scanning unit. In both cases a tensioning device holds the band properly in position.
Frequently, such length measuring devices are used to measure relative displacements, and often they must be attached to appropriate machine components. When such machine components are included in machine tools, operation of the machine tool can generate severe interfering influences related to temperature changes and contamination.
The above described tensioning devices are provided to reduce measurement errors associated with temperature influences. In order to protect against contamination, the measuring devices disclosed above are encapsulated, for example in a sealed hollow body. A further possibility lies in the attachment of the length measuring device at a protected region of the machine tool. However, complete protection against fouling cannot be achieved without raising capital expenditures excessively. For this reason periodic cleaning of the sensitive measuring device is typically required. Such cleaning of the measuring device can be carried out during routine maintenance of the machine tool to which the measuring device is attached.
Depending upon the position in which the length measuring device is attached to the machine and the precise structure of the housing, the disassembly of length measuring devices can represent a major maintenance problem. Thus, by way of example, when the apparatus shown in U.S. Pat. No. 3,816,002 is to be disassembled, the housing has to be opened, the tensioning device of the measuring band has to be released, and then the scanning component has to be removed. Assembly is performed in reverse sequence and in addition the measuring band must be recalibrated and readjusted after assembly has been completed.